1. Field of the Invention
The invention relates to a device for testing a test specimen for surface faults by magnetization means by delivering a magnetic flux into the specimen and using induction probes as measurement sensors, the induction probes being capable of turning around the specimen, which can be guided in the lengthwise direction, in order to scan it helically.
2. Description of Related Art
These devices are used to determine, for example, surface faults on metallic articles with a round profile, such as billets, bars, steel pipes and the like, while these articles, which can have high temperatures depending on the choice of fabrication process, run through a production line, for example, a mill train.
Magnetization means magnetize the test specimen such that the faults near the surface produce a stray flux which can be detected by means of induction probes. Faults in and under the surface of a test piece cause, for example, a stray flux which is detected by means of induction probes. Therefore, the output signals of the induction probes are a measure of impurities and faults in and under the surface of a test specimen. The induction probes for fault detection are located near the peripheral wall of the test specimen. The induction probes can be suited to detecting magnetic stray flukes or can be designed as eddy current sensors. The induction probes can also operate based on magneto-resistive effects.
German Patent DE 39 37 261 C2 and corresponding U.S. Pat. No. 5,187,435 describe such a device, with a rotating head, for scanning of metallic test material. The rotating head is composed essentially of a housing having a hollow shaft for routing through a test specimen, and having a rotating part to which the induction probes, for example, the eddy current detectors, are attached. Furthermore, there are drive means to produce the peripheral motion of the rotating part. The drive wheel of a drive motor is connected via a drive belt to a drive wheel which sits rigidly on the hollow shaft. On each of the front and back end of the hollow shaft sits a respective protective sleeve, by which the test specimen is guided. A rotating disk is rigidly connected to the hollow shaft. Two levers are pivotally mounted on the hollow shaft and each bears an induction probe. A magnetic flux in the lengthwise direction is routed into the test specimen by means of a magnetization coil which surrounds it and by means of a magnetic flux conductor. Parts of the housing and the hollow shaft are used as the magnetic flux conductor. To homogenize the magnetic field in the test region, there is a homogenization ring of magnetically conductive material.
German Patent Application DE OS 29 05 399 and corresponding UK Patent Application 20 14 317 application disclose another such device and in which a test specimen can be guided by a cylindrical element which can be rotated around its lengthwise axis. Several induction probes, which are made as eddy current fault detector coils, are attached to the cylindrical element near its peripheral wall. By rotating the cylindrical element and simultaneously guiding the test specimen in the lengthwise direction, the specimen is scanned helically by the coils. The output signals of the coils are evaluated in a signal detector circuit. An alternating current signal of fixed frequency is applied to the coils and produces an eddy current a short distance underneath the surface of the test specimen. Faults in the test specimen change the coil impedance. The magnetic flux runs in the test specimen in the lengthwise direction on and underneath the jacket surface of the specimen.
The indicated known devices are subject to the first disadvantage that they cannot be quickly and economically adapted to different dimensions, especially the cross sections, of the test specimens. A second disadvantage lies in the inadequate heat dissipation which, as the third disadvantage, sets narrow limits on the service life of the device, and finally, limits the magnetizing power as the fourth disadvantage. The shape of the coil cores used in the indicated devices can only be changed at high costs for bobbin cores which have been wound insulated.